Diaphragm electrolytic cells are well known and widely employed in the production of chlorine and caustic. Customarily, the diaphragm is formed from asbestos fibers which are deposited on a foraminous structure, such as a cathode, by drawing the diaphragm from a slurry onto the surface of the cathode member. The life of a diaphragm has heretofore presented no particular problem because the graphite anodes which were widely used needed to be replaced before the diaphragms. When the electrolytic cells were down for replacement of the anodes the diaphragms were also replaced. The new metal anodes now being used commercially, however, have a life substantially longer than the graphite anodes, thus making longer lasting diaphragms desirable. Furthermore, the small clearances between the metal-type anodes which are interdigited with the finger-type cathodes make the use of asbestos diaphragms difficult due to the bridging of the narrow gap between the electrodes by asbestos fibers. An additional reason for considering asbestos-free diaphragms is the environmental concern about the use of asbestos.
One solution to this particular problem is the use of preformed diaphragms made of various plastic materials. These diaphragms are normally manufactured in sheet form and must be fitted about one electrode, either the anode or the cathode, in order to separate the catholyte and anolyte compartments. Patents disclosing such preformed diaphragms made of various types of plastics and containing various fillers, pore-formers, etc., include U.S. Pat. Nos. 3,930,979; 4,020,235; 4,036,729; 4,089,758; 4,098,672; 4,126,536; 4,170,540; and 4,184,939.
U.S. Pat. No. 4,210,515 describes, as one alternative form of the invention claimed, a diaphragm which is vacuum deposited from a slurry composed of discrete fibers of a self-bonding thermoplastic material and a surfactant. One modification suggests that talc and other inorganic wetting aids may be incorporated into the plastic material.
U.S. Pat. No. 4,311,566 describes a method for making a diaphragm for electrolytic cells wherein a slurry is made of non-asbestos, fibrous material, a binder and a pore-forming material. The slurry is drawn through a foraminous electrode to produce the diaphragm. The pore-forming material is removed, either by destruction through heating or through solution.